Positive type photoresist compositions are normally compositions comprising an alkali-soluble resin and a naphthoquinonediazide compound as a light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinone diazide-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. The most common examples of such compositions include a novolak resin made of cresol formaldehyde/trihydroxybenzophenone-1,2-naphthoquinone diazidosulfonic ester, as disclosed in L. F. Thompson, "Introduction to Microlithography", ACS, No. 219, pp. 112-121.
A novolak resin, which is a binder, can be dissolved in an aqueous alkaline solution without swelling. A novolak resin can also exhibit a high resistance, particularly to plasma etching, when an image thus produced is used as a mask for etching. Thus, a novolak resin is particularly useful in this application.
As a photosensitive material, a naphthoquinone diazide compound serves as a dissolution inhibitor in order to reduce the alkali solubility of the novolak resin. Such a compound is peculiar, however, in that it undergoes decomposition upon irradiation with light and produces an alkali-soluble substance which rather enhances the alkali solubility of the novolak resin. Because of the great change in properties resulting from irradiation with light, a naphthoquinone diazide compound is particularly useful as a photosensitive material for positive type photoresist.
From this standpoint, many positive type photoresists comprising a novolak resin and a naphthoquinone diazide photosensitive material have heretofore been developed and put to practical use. These positive type photoresists have attained sufficient results in working lines of a width of 1.5 to 2 .mu.m.
However, even with the use of such positive type photoresists having a high resolution, if the patterns are formed on the surface of a substrate having a high reflectance, such as an aluminum substrate, a dim image is formed and it is very difficult to control the width of the lines which form the patterns, due to the influence of the light reflected by the surface of the substrate, which is known as halation. This phenomenon is more remarkable in the case where the substrate physically has different levels or steps.
For the purpose of solving such drawbacks, i.e., preventing halation, it has been known to incorporate a light absorbing material into the photoresist composition. For example, JP-B-51-37562 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes a method in which the transmitting capability of the photoresist layer is reduced by incorporating therein a dye, Oil Yellow, having light absorption characteristics in the ultraviolet region, said dye being represented by the formula: ##STR3## This results in the reduction of the amount of light which is reflected by the surface of the substrate and which passes through the photoresist layer, and thereby reduces the amount of light which is transmitted to the area that is not to be exposed to ultraviolet rays; whereby the resolution is prevented from being deteriorated.
However, when this dye is incorporated into the photoresist composition, a part of the light absorbing material present in the photoresist composition sublimes from the photoresist layer when a substrate coated with the photoresist composition solution is subjected to prebaking, which serves the purpose of removing the residual solvent in the coating and improving adhesion between the substrate and the coating. Consequently, the halation-preventing capability of the photoresist is considerably attenuated, and the resist performances, such as sensitivity or the like, become inconstant.
U.S. Pat. No. 4,287,289 describes derivatives of a light absorbing agent (1-alkoxy-4-(4'-N,N-dialkylaminophenylazo)benzene in which subliming characteristics during prebaking are improved. However, when such a light absorbing agent is incorporated into a common positive type photoresist composition, sensitivity is considerably attenuated.
JP-A-59-142538 (the term "JP-A" as used herein means an "unexamined punished Japanese patent application") describes an alkali-soluble azo compound. When this compound is used, the halation-preventing capability obtained is not fully satisfactory although the attenuation of sensitivity and the variability of sensitivity due to the nature of the light absorbing material used are low, and failing to meet the recent requirements of considerable miniaturization of working dimensions required in the semiconductor industry.
JP-A-1-241546 describes a system of a combination of an ultraviolet absorbing agent and a gallic acid ester or polyhydroxybenzophenone and a 1,2-naphthoquinone diazide and/or 1,2-naphthoquinone-4-sulfonic ester. However, such a system does not provide a sufficient halation-preventing capability.